The working conditions of the injection mold runner outlet are harsh and prone to wear, which causes changes in its shape and size, and has a great impact on the quality and production efficiency of the product. Based on this, this study uses laser cladding technology to locally strengthen the injection mold runner outlet, and conducts research on the design of the strengthening coating material composition, cladding forming and defect control, organization and performance analysis, and actual workpiece processing technology regulation. The results show that compared with commercial wear-resistant coating materials, the developed coating material has excellent cladding forming and no crack defects. The hardness is increased from 400HV of the substrate before strengthening to more than 900HV, the wear resistance of the coating is increased by 43 times compared with the substrate, and the friction coefficient is reduced from 0.635 of the substrate to 0.412, which greatly improves the wear resistance of the mold; the internal grains of the strengthened coating are fine, and its wear resistance is mainly guaranteed by martensite and in-situ ceramic carbides, and a small amount of residual austenite guarantees its toughness; through special path planning and process control, it has been successfully applied in the local strengthening of injection molds, and the service life of the mold after strengthening is increased by more than 3 times. (Injection mold; Laser cladding; Microstructure; Wear resistance)

1 Introduction
Plastic products are widely used in aerospace, electronics, machinery, automobiles and people’s daily life. Injection molds are indispensable tools in the production process of plastic products. Injection molding molds undergo thousands of injection molding processes every day. The working conditions of the injection molding runner outlet are relatively harsh. They are in a service environment of repeated friction and corrosive media, which is very easy to wear and corrode, causing the shape and size of the injection molding runner outlet to change, which greatly affects the quality of the product and the stability of the production process. In severe cases, the mold will be directly scrapped, causing huge losses to the company [1-2]. Therefore, it is necessary to strengthen the gate part of the injection mold to improve the service life of the mold and ensure the quality of product production. Laser cladding technology is highly flexible and can be used to locally strengthen parts with complex structures and high dimensional accuracy. The bonding strength between the cladding layer and the substrate is high, and the substrate damage is small. It is more suitable for local strengthening of precision injection molds [3]. H13 steel has good thermal strength, toughness and thermal fatigue resistance, and is widely used in the mold industry [4,5]. Researchers have also used laser cladding to improve the wear resistance of H13 steel. First, researchers have used composite coatings reinforced with ceramic particles to improve wear resistance, mainly including TiC/H13 composite coatings [6] and Stellite-WC composite coatings [7]. These composite coatings can significantly improve the surface wear resistance of H13 steel and the service life of the mold. However, the ceramic particle reinforcement phase is easily ablated and forms holes during the cladding process, which will have a great impact on the surface quality of injection mold products. Secondly, researchers have clad heterogeneous alloy materials on the surface of H13 steel to improve wear resistance. Norhafzan prepared NiTi coating on the surface of H13 steel, and its hardness is 2.9 times that of H13 steel [8]. Liu Hongxi prepared self-melting Ni-based coating on the surface of H13 steel, and its wear resistance was improved by 3.8 times [9]. These coatings significantly improve the surface wear resistance of H13 steel, but the thermal expansion coefficients of dissimilar materials and H13 are quite different, which makes the heat dissipation of different positions of the product after injection molding very different, which has a great impact on the quality of plastic products. Based on the above analysis, the preparation of iron-based coating materials on the surface of H13 steel becomes the preferred choice for injection mold reinforcement. Cao Jun prepared Fe104 alloy coating on the surface of H13 steel, and the wear resistance of the steel increased, but the coating was easily affected by the release agent during the demolding process and failed [10]. Ren Jianhua prepared CPM9V cladding layer on the surface of H13 steel, and the hardness and wear resistance were improved, but it was still difficult to meet the requirements of high-frequency injection molding conditions [11]. Feng et al. clad the surface of the substrate with a highly wear-resistant iron-based coating material, and the wear resistance was significantly improved. However, cracks were easily generated inside the coating. Reducing stress can help reduce coating cracks [12]. According to the above analysis, although the laser cladding technology can increase the surface hardness and wear resistance of the material, when the hardness is too high, cracks are likely to occur in the cladding layer. Although the preheating method can reduce the sensitivity of cracks, this increases the complexity of the process and reduces the efficiency, and it is difficult to be applied in the local strengthening of precision injection molds. Therefore, how to ensure that the coating has high hardness and wear resistance, and does not produce cracks without preheating, has become the key to the local strengthening of injection molds. This study will make breakthroughs in both materials and processes.

2 Design and test process of wear-resistant materials
In order to maximize the wear resistance of injection molds and extend the service life of injection molds, a high-wear-resistant and high-toughness iron-based coating material is specially designed. Through the regulation of elements, the wear resistance and crack resistance of the cladding coating are improved at the same time. Among them, Cr element has a solid solution strengthening effect on the matrix, and the addition of Nb, V, and Ti will form a hard particle phase in situ, further improving the wear resistance of the cladding layer. At the same time, the addition of elements such as Fe and W will form a eutectic precipitation phase, which will improve the hardness of the cladding layer and also play a certain crack resistance. The elements and composition of the designed components are shown in Table 1 for coating 3. The matrix material used for the injection mold is H13 mold steel, and the specific composition is shown in Table 1. In addition, commonly used commercial wear-resistant stainless steel coating materials are selected for comparison with the materials designed in this study. Among them, coating 1 is R401 powder produced by Hegenas (China) Co., Ltd., and coating 2 is FJ-21 powder produced by Tianjin Zhujin Technology Development Co., Ltd. The specific composition is shown in Table 1 for coatings 1 and 2. Figure 1 shows the laser cladding equipment. During the cladding process, the laser used is LDF5000, the beam is a circular spot with a diameter of 1.2mm, the laser power is set to 1400W, the cladding speed is 1.5m/min, the powder feeding rate is 1.2r/min, and the overlap rate between two passes is 50%. Through the planning of the beam path, the preparation of wear-resistant coatings on the mold substrate material with cladding powders of different components is realized. After the cladding is completed, the formation and microstructure of the cladding layer are observed and analyzed using optical microscopy (OM) and electron backscattering (EBSD), and the hardness and friction and wear properties of the coating are tested. Among them, the friction and wear equipment is the multi-functional friction and wear tester MFT-5000 produced by Aitek Instrument Technology. The test process is shown in Figure 2.

3 Cladding Forming and Microstructure Analysis of Coating Materials

The laser cladding forming of coating materials with different compositions on the die steel substrate is compared, and the results are shown in Figure 3.

According to the results Figures 3(a) and 3(c), when laser cladding is performed using coating material 1 and the developed coating material 3, the cladding layer is well formed, the surface is smooth and continuous, and from the cross section of the joint, there are fewer internal defects, the cladding layer and the substrate are well bonded, and no defects such as cracks are found at the interface.

However, after laser cladding using coating material 2, it can be directly observed from the surface that many cracks perpendicular to the cladding direction are formed in the cladding layer, and from the cross section of the joint, the cracks almost penetrate the entire cladding layer.

The cracks are formed during the cladding process or just after the cladding is completed.

The formation process is accompanied by a crisp sound, showing typical cold crack characteristics. Therefore, the toughness and crack resistance of coating material 2 are relatively poor.

The microstructure and phase formation of the coating were further analyzed. Since the content of alloy components C and B in coating material 2 is high and the hardenability is strong, a large amount of high-hardness martensitic structure will be produced during the cladding process. However, material 2 has fewer austenite and carbide forming elements, and the adjustment of organizational plasticity is limited. Cracks occurred after cladding. It will not be analyzed here. Only coating material 1 and the developed coating material 3 are analyzed. The results are shown in Figure 4. Through the comparative analysis of Figures 4(a) and 4(b), when coating material 1 is used for laser cladding, the microstructure shows obvious columnar crystal growth and large grain size; however, when coating material 3 is used for laser cladding, under the same heat input and heat dissipation conditions, the grains are fine, no obvious columnar crystal formation occurs, and more precipitation phases are formed inside the cladding layer. The phase composition of coating materials 1 and 3 was further analyzed, and the proportion and size of the formed phases were statistically analyzed. The results are shown in Figure 5 and Table 2. According to the results, the cladding coating formed by material 1 is composed of a large amount of blocky martensite and residual austenite at the boundary. A small amount of small carbide particles are precipitated on the coating. Among them, martensite makes the coating have higher hardness and wear resistance, and residual austenite ensures that the coating has a certain toughness and inhibits the generation of cracks. In the coating formed by material 3, the amount of martensite is reduced, but a large amount of carbide ceramic phase with higher hardness is formed, which makes the coating have better wear resistance. In addition, in the developed coating material 3, the addition of elements changes the morphology and size of the martensite and carbide ceramic phases in the coating. In particular, the addition of W elements will form a eutectic phase composed of carbides and Fe coating matrix, which ensures that the coating has good wear resistance and toughness and improves the coating’s crack resistance. In addition, a small amount of austenite remains in the coating, which also helps to inhibit the generation of cracks in the coating. 4 Performance test analysis of different coating materials
The hardness test of cladding coatings with different compositions was carried out, and the results are shown in Figure 6. It can be seen that the average hardness of the substrate is 400HV, and the average hardness of coating components 1, 2 and 3 are 600HV, 800HV and 900HV respectively. Although coating 1 has no cracks, its hardness is not high; compared with commercial coating material 2 and developed coating material 3, the hardness of material 2 is lower than that of material 3, and the cladding coating formed by material 2 has cracks, which shows that the developed coating has good strength and toughness.

Friction and wear tests were carried out on cladding coatings with different compositions, and the wear marks after the test were observed and analyzed. The results are shown in Figure 7 and Table 3 respectively.

According to Figure 7, the substrate forms wide and deep grooves in the wear test, the depth of the grooves of coating 1 is significantly reduced, the depth of the grooves of coating 2 is reduced, and the grooves of coating 3 are narrow and shallow. According to the results obtained in Table 2, in terms of average wear volume, coatings 1 and 3 are 3 times and 43 times higher than the substrate, respectively, and the friction coefficient is also lower than that of the substrate. The friction coefficients of the substrate, coatings 1, 2 and 3 are 0.635, 0.5739, 0.493 and 0.412, respectively.

5 Local reinforcement of injection molds
Injection molds are essential tools for the production of various plastic products and are widely used in industrial production. Figure 8 shows a type of injection mold. During the use of the mold, the central injection port is in harsh service conditions and is prone to wear, resulting in mold failure, affecting product quality and production efficiency. It is necessary to locally strengthen the injection port.

The specific strengthening process is shown in Figure 9. First, as shown in Figure 9 (a), a spherical crown groove is machined at the mold port by machining. The diameter of the groove opening is 4 mm, the depth of the groove is 2 mm, and the distance between the bottom of the groove and the top of the casting channel is 1.5 mm. Second, as shown in Figure 9 (b), according to the shape of the groove, a laser cladding additive model is established to plan the relevant printing path. Among them, the laser scanning path is circular, specifically, it expands outward from the center of the bottom of the groove; the additive process is completed in three layers, including first laser scanning 1-2 circles at the bottom of the groove to form a wear-resistant bottom layer, then laser scanning 2-4 circles on the surface of the wear-resistant bottom layer to form a wear-resistant middle layer, and finally laser scanning 4-6 circles on the surface of the wear-resistant middle layer to form a wear-resistant upper layer; the laser power is set to 1400W, the cladding speed is 1.5m/min, the powder feeding rate is 1.2r/min, the overlap is 0.5mm, the laser head lifting amount is 0.5mm, the powder feeder flow is 12L/min, and the shielding gas flow is 20L/min; in addition, according to the previous analysis of the formation and performance of the cladding coating obtained with different components, the developed coating material 3 is used to perform local cladding strengthening of the mold. The mold is fixed at the printing station, and the laser spot is adjusted to the cladding starting point, and the program is run to complete the local strengthening process of the mold. Finally, as shown in Figure 9 (c), the excess material is removed by machining, and a cylindrical injection port with a diameter of 1mm and a depth of 1.5mm is formed on the wear-resistant layer, so that the injection port is connected to the injection channel in the mold, and it is polished to obtain a partially reinforced injection mold. The developed process and materials are used to strengthen the experimental piece. The section after strengthening is shown in Figure 10. It can be seen that the inside of the strengthening layer meets the requirements and is well combined with the base material. After testing, the hardness and wear resistance meet the requirements. Finally, this technology is applied in real mold strengthening. The results are shown in Figure 11. After the injection molding manufacturer evaluates the service life of the strengthened mold, compared with the unstrengthened mold, the service life of the strengthened mold is increased by more than 3 times, which greatly improves the life and maintenance cycle of the mold, reduces the cost of the enterprise, improves the production efficiency of the enterprise, and ensures the high-quality production of products. 6 Conclusion
This paper has achieved defect-free preparation of high wear-resistant coatings by strengthening the design of coating material composition, cladding forming and defect control, and microstructure performance analysis. The conclusions are as follows:

1) Based on traditional commercial wear-resistant coating materials, by regulating the alloying elements of the materials, mainly by adding and controlling the proportion of elements such as W and B, a wear-resistant coating mainly composed of martensite and eutectic in-situ ceramic carbides was obtained. A small amount of austenite remains in the coating, which makes the coating have good toughness and no cracking occurs during the cladding process.

2) Compared with the base material and traditional commercial coating materials, the hardness of the developed wear-resistant material exceeds 900HV, the wear resistance of the coating is improved by 43 times relative to the base, and the friction coefficient is reduced from 0.635 of the base to 0.412, which greatly improves the wear resistance of the mold.

3) By designing a special concave groove at the mold gate, performing three-layer multi-channel path control and strict energy regulation during the cladding process, the defect-free preparation of high wear-resistant coatings was successfully achieved at the mold gate, which increased the service life of the mold to more than 3 times the original, which is of great significance for ensuring the quality and processing efficiency of the company’s processed products and reducing the company’s costs.